Method of preparing a fuel element for a nuclear reactor



element for a nuclear reactor.

United States Patent Ofiice 2,900,263 Patented Aug. 18, 1959 METHOD OFPREPARING A FUEL ELEMENT FOR A NUCLEAR REACTOR Joseph H. Handwerk,Juliet, and Roland A. Bach, La Grange, Ill., assignors to the UnitedStates of America as represented by the United States Atomic EnergyCommission No Drawing. Application February 13, 1957 Serial No. 640,056

3 Claims. (Cl. 106-39) This invention relates to a method of preparing afuel More particularly the invention relates to a method of preparing acorrosionresistant ceramic reactor fuel element.

Ceramic materials such as uranium dioxide (U and thorium dioxide (ThOhave been suggested for use in nuclear reactors because of theirstability under oxidizing conditions, good corrosion resistance, lack ofundesirable phase transformations and high fusion point. Fuel elementsfor nuclear reactors containing a mixture of thorium dioxide and uraniumdioxide are disclosed in patent application Serial No. 612,906, filedSeptember 28,'1956, on behalf of John M. West and James F. Schumar, nowPatent No. 2,838,452, issued on June 10, 1958. Fuel elements of thistype are suitable for use in water-moderated reactors wherein the wateris heated by the fuel to form steam. Reference is made to Untermyei'application Serial No. 518,427, filed June 28, 1955, for a completedescription of details of construction and operation of a reactor ofthis type.

Mixtures of thorium dioxide and uranium dioxide are employed becauseuranium dioxide alone is not very satisfactory as a reactor fuel in someof the more common types of reactors. For example, uranium dioxidebodies are not satisfactory for use in the aforementioned Untermyerreactor because of the possibility that the moderator might penetratethe cladding for the fuel elements. If the moderator did penetrate thefuel element, as would be quite likely to happen at some time during thelife of the reactor, the uranium dioxide would disintegrate.

Uranium dioxide fuel is not very satisfactory in any reactor in which itis or may be subject to an oxidizing atmosphere. In a water-cooledreactor the oxidizing atmosphere exists because of the effect ofradiation on the water used as moderator. for the disintegration of theuranium dioxide is that it has a tendency to oxidize to U 0 in anoxidizing atmosphere, and this change in form is accompanied by anincrease in volume. Thorium dioxide on the other hand does not have thistendency to oxidize since it forms no higher oxide. It has also beenfound that mixtures of thorium dioxide and uranium dioxide are notseriously affected even if the moderator does penetrate the housing. Useof thorium dioxide along with the uranium dioxide offers the additionaladvantage that additional fissionable material may be bred from thethorium.

Thus we see that ceramic fuel elements and particularly those containinga mixture of uranium dioxide and thorium dioxide are very important atpresent and likely to become more so in the future. Unfortunately,difficulties have. been encountered in the preparation of ceramicscontaining a mixture of uranium dioxide' and thorium dioxide. When amixture of thorium dioxide and uranium dioxide is fired in an oxidizingatmosphere such as air, the uranium dioxide in the mixture will beoxidized to U 0 at temperatures below 700 C. and

It is believed that the cause 2 then reduced back to uranium dioxideabove 700 C The oxidation to U 0 is accompanied by a volume-increasewhich ruptures the body. Previous workers in the art have restrictedtheir attention-to methods of protecting ceramic fuel elements composedof U0; and ThO against oxidation of the U0 by procedures in which themixture is fired in a high vacuum or in a highpurity hydrogenatmosphere. All of these methods had one objective in view-prevention ofthe formation of U 0 by oxidation of U0;. These methods result insatisfactory fuel elements but only at greatly increased cost because ofthe difliculty in carrying out such' procedures.

It would therefore be extremely desirable to develop'a method forpreparing ceramic fuel elements containing uranium dioxide and thoriumdioxide whereby the ceramic can be prepared by firing a mixture ofreadily available constituents in air, and this is the object of thepresent invention.

This object of the invention is satisfied by our novel method ofpreparing ceramic fuel elements containing thorium dioxide and uraniumdioxide which comprises forming a mixture of thorium dioxide and anoxide of uranium in which the uranium ispresent in an oxidation statehigher than it is in uramum dioxide into the desired size and shape andfiring in air at a temperature sufliciently high to reduce the higheroxide of uranium present therein to uranium dioxide. The selection of ahigher oxide offers an additional advantage over the use of U0;, since ahydrogen reduction step is saved because U0,- is made from the higheroxides such as U 0; and U0, by hydrogen reduction.

We have found surprisingly that a mixture of thorium dioxide and U 0 orother higher oxides of uranium can be fired in air to obtain a ceramicbody which is suit-. able for use as a nuclear reactor fuel element.This is the direct opposite of the prior art methods previously referredto since we use U 0 as a starting material while the previous workers inthe art went to great pains to prevent the formation of U 0 It isbelieved that the reason that a satisfactory fuel element can beprepared in this manner is that the U 0 or other higher oxide such as U0decomposes to uranium dioxide'at temperatures above 700 C. Thedecomposition results in shrinkage of the body or the formation of voidstherein. The decomposition takes place at a temperature sufiicientlyhigh so that internal stresses resulting from shrinkage of the body orthe formation of voids therein are relieved without cracking.

As the higher oxide is reduced to uranium dioxide, it dissolves in thethorium dioxide forming a solid solution therewith causing the reactionto proceed more nearly to completion.

It is important that the firing temperature be sufficiently high so thatall or substantially all of the higher oxide of uranium be reduced touranium dioxide. The minimum temperature required depends on thepropor'-- tion of uranium within the mixture. For a mixture of U 0 andthorium dioxide containing only 1% uranium dioxide (calculated), thefiring temperature must be above about 1450 C. The preferabletemperature, however, would be over 1700 C. to obtain faster reactionspeed. As the proportion of uranium dioxide increases the preferabletemperatu e will increase up to around 2000 C.-

The maximum temperature permissible is, of course, determined by thefusion temperature of the mixture. In general, our novel methodcomprises intimately mixing finely divided U 0 or U0 with thoriumdioxide,'

pressing the mixture to form a body of the desired size and shape, andfiring the body at a temperature between, 1450" C. and the fusion pointof the mixture to obtain' a ceramic body suitable for use as a fuelelement in a nuclear reactor.

Mixing of the mixture must be complete. Mixing of the mixture can beaccomplished either by wet milling a thermal neutron flux of 2--3 10neutrons/cm? sec. known as CP-S and in an enriched-uranium,light-watermoderated research reactor having a thermal neutron flux of34 10 neutrons/cm. sec. known as MTR.

or by dry milling. The procedures are as follows: 5 The tests in CP-Swere carried out in pressurized water (1) Water and a small amount of abinder is added to autoclaves at a temperature of SOD-520 F. and apresthe required quantity of finely ground uranium oxide sure of 680-810p.s.i. and the tests in the MTR were and thorium dioxide; the mixture iswet milled and the carried out in capsules containing NaK. water removedby drying; the composition is then re- The table following gives thedetails of these tests.

Table Fired t- Wt. Percent 1 Time of U0; Reactor Te'st, hrs. Iacketing lBonding Environment Condition Temp. of- Deifislty C. GJcc.

1. 9.58 (JP- 440 Bare None Pressurized water-.. Slight crack. 1. 9.58CP-5 1,100 Zirca1loyII do do Some fragmentation. 1. 9.58 CP-5 1.100 Type347 stainless steel do do Do. 1 9.58 M'IR 3,000 Bare do N K Sound. 19.58 M'IR 3,000 Al1% Ni- Nalr (in 1 9.58 MIR 3,000 Type 347 stainlesssteel None dn Do. 1 9.58 MTR 3,000 Zircalloy II- do (In 9.2 CP-5zoo-1,590 Al1% Ni Lead.... Pressurized water-.. Intact but fragmented.9.2 OP-5 1,042 Zircalloy II- rln (in Do. 10.06 MTR 1,400 A.11%N (in N21?Sound. 1,800-1, 830 10.06 MTR 1,400 do Gas .10

1 Time is calculated time at full power of reactor tempered by theaddition of about 2% water and is pressed into the required size andshape; and the resulting body is fired at a temperature sufficientlyhigh to reduce the uranium oxide to uranium dioxide.

(2) The mixture of uranium oxide and thorium dioxide is dry milled; asmall quantity of water is added thereto and the mixture is granulatedby passing it through a screen; a binder is added and the mixture isformed into shape by pressure; and the resulting body is fired asbefore.

A mineralizer or sintering aid such as calcium oxide,

magnesium oxide, or calcium fluoride may be used to obtain betterdensification and to avoid the use of high forming pressures and highfiring temperatures.

. In addition to pressing the fuel bodies may be formed by slip casting.See The Preparation of Refractories from Uranium Oxide, J. Am. CeramicSociety, 36 (4), 137-40 (1953), for a discussion of this subject.

A number of compositions containing U 0 and ThO were prepared andexamined visually. All specimens were prepared by wet milling and drypressing into right cylinders A in length. The compositions were firedto l7001750 C. in air. Ware containing up to weight percent U 0 was freeof visible defects and underwent no visible changes when reheated in airto 1400 C. for periods up to 96 hours. The geometric densities of thesesamples ranged from 7.85 gm./cc. for compositions containing 2 /2 weightpercent U0 (added as an equivalent amount of U 0 to 7.00 g./cc. forcompositions containing 30 weight percent U0 (added as an equivalentamount of U 0 Compositions containing between 30% and 70% U0 weresomewhat cracked but indications are that they would be satisfactory foruse as fuel elements.

Since these tests showed that it is possible to prepare good ceramicbodies by firing a mixture of U 0 and ThO in air, a number of suchbodies were prepared and irradiation tests made thereon. The ceramicbodies prepared were cylinders .25" in diameter. The bodies containing2%% U0; were .375 inch long while the bodies containing 10% U0 were .50inch long. All of the bodies were mixed by wet milling. All samples werefired in air.

The irradiation tests were carried out in an enricheduraniumheavy-water-moderated research I63 a ing Post irradiation examination ofthe samples showed that all were intact although a few had fragmentedand others had microscopic cracks therein. Dimensional changes, if any,were slight under the test conditions and fission products wereapparently well contained within the specimen capsules. The samples weresubjected to suflicient thermal cycles to make it evident that theywould stand up under a reasonable number of temperature changes.

These preliminary tests indicated that a thorium dioxide-uranium dioxidesolid solution fuel would be feasible and that the ceramic materialcould be fabricated with little or no ditliculty.

Accordingly, it was decided to employ fuel elements prepared asdescribed herein as the fuel for the lightwater-moderated,enriched-uranium research reactor known as Borax-IV. The composition ofthe mixture employed was 93.41 weight percent thorium dioxide and 6.59weight percent U 0 The U 0 used was above 90% enrichment in U Both thethorium dioxide and U 0 employed were --325 mesh. The thorium dioxidewas calcined overnight at 1000 C. to improve its pressingcharacteristics.

The U 0 and thorium dioxide were mixed by dry milling in aporcelainlined pebble mill employing porcelain pebbles with 2 /2 weight percentof the batch of polyvinyl alcohol added as a binder. The constituentswere mixed for three hours.

The dry powder was dampened with 12 weight percent of water containing0.5 weight percent of an aerosol. The moist mixture was forced through a16 mesh screen to form granules and the granulated material was dried atC. for four hours.

Before pressing 1 weight percent of a mixture of 50 weight percentkerosene and 50 weight percent oleic acid was added as a lubricant. Thematerial was pressed in a double acting press at 14,000 p.s.i. Thediameter of the resulting pellets was controlled by the die size andvaried from-67" to .270". The length of the pellets was controlled bythe free flowing characteristics of the granulated material and variedbetween .750" and .375".

The pellets were heated overnight at 260 C. in alurnina crucibles in anelectric furnace to remove most of the organic binder. They were thentransferred to a gas fired furnace where they were fired in air to 1750"C. in

approximately 16 hours. The peak temperature was held for two hours, andthe ware was allowed to cool in the furnace to room temperature.

The fired ware was found to have an average geometric density of 8.3g./cc. and an average apparent density of 9.8 g./cc. The fired ware ingeneral was of uniform appearance and rejects averaged less than 4%.

The ceramic pellets resulting were loaded into subassemblies of the typedisclosed and claimed in the patent application of West and Schumarpreviously referred to, and the subassemblies were employed as fuel forBorax- IV which has gone critical and is undergoing tests preparatory toplacing it in full operation.

Thus it is apparent that fuel elements prepared as describedhereinbefore are satisfactory and that our invention makes it possibleto prepare them easily and cheaply.

It will be understood that this invention is not to be limited to thedetails given herein but that it may be modified within the scope of theappended claims.

What is claimed is:

1. A method of preparing a reactor fuel element which comprises forminga mixture of thorium dioxide and an oxide of uranium in which theuranium is present in an oxidation state at least as high as it is in U0 into a desired shape and firing in air at a temperature sufiicientlyhigh to reduce the higher uranium oxide to uranium dioxide.

2. A method of preparing a reactor fuel element which comprisesintimately mixing finely ground U 0 and thorium dioxide, the resultingmixture containing not over weight percent U 0 pressing the mixture intoa compact mass, and firing in air at a temperature above 1450 C., themass after firing containing not over 70 weight percent U0 added as anequivalent amount of U 0 3. A method of preparing a reactor fuel elementwhich comprises intimately mixing 6.59 weight percent, --325 mesh U 0and 93.41 weight percent thorium dioxide, pressing the mixture into acompact mass at 14,000 p.s.i., and firing in air at a temperature of1750 C.

References Citedin the file of this patent 'UNITED STATES PATENTS Sheftet a1. June 10, 1952 OTHER REFERENCES

1. A METHOD OF PREPARING A REACTOR FUEL ELEMENT WHICH COMPRISES FORMING A MIXTURE OF THERIUM DIOXIDE AND AN OXIDE OF URANIUM IN WHICH THE URANIUM IS PRESENT IN AN OXIDATION STATE AT LEAST AS HIGH AS IT IS IN U3O3 INTO A DESIRED SHAPE AND FIRINF IN AIR AT A TEMPERATURE SUFFICIENTLY HIGH TO REDUCE THE HIGHER URANIUM OXIDE TO URANIUM DIOXIDE. 